Retinal rods and cones utilize visual pigments that are coupled to G-protein signaling cascades to detect the presence of photons. Little is known about how these two cell types use such a remarkably similar phototransduction cascade to achieve their distinct functional properties. A major objective of this project is to address this fundamental question. An understanding of the mechanisms that regulate phototransduction also has important disease relevance because deregulated signaling that occurs at different transduction steps often has a negative impact on photoreceptor cell survival. Our second objective is to apply what we learned in phototransduction and translate that knowledge into a better understanding of disease mechanisms in rods and cones so that a rational therapeutic strategy can be devised. In the first aim, we will test the hypothesis that differences in the functional properties between rods and cones can be explained, in part, by the transduction efficiency between the visual pigment and transducin, the visual G-protein. This hypothesis is supported by our preliminary results that show a 100-fold decrease in sensitivity when cone transducin was placed downstream of rhodopsin. Experiments in Aim 1 will systematically analyze the contribution of the rod and cone isoforms of the heterotrimeric transducin subunits toward the observed decrease in transduction efficiency at this step. The potential function of the G[unreadable]? subunit in controlling photoreceptor noise and response recovery will also be investigated. In the second aim, we will investigate the cell death pathways that are triggered by light exposure or genetic mutations that lead to "equivalent light". Experiments in Aim 2 are designed (a) to probe the underlying mechanism for the toxicity of the rhodopsin/arrestin complex;(b) to investigate whether endocytosis of the rhodopsin/arrestin complex is required to generate the cell death signal;(c) to analyze whether constitutive phototransduction in cones is a potential mechanism for cell death;and (d) to study the involvement of ATF-3 and ATF-4 transcriptional regulators in the cellular response to light damage. The outcome from these experiments will address the fundamental question as to how cells may utilize G-protein signaling cascades, which consist of highly similar protein members, to achieve diverse signaling properties. In addition, we will gain a better understanding of the relationship between defective signaling and disease.